aboutsummaryrefslogtreecommitdiffstats
path: root/plumbing/format/packfile/diff_delta.go
blob: d35e78aead7f77bdd3cc6c63a2c9f95f5c191ce3 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
package packfile

import (
	"bytes"

	"gopkg.in/src-d/go-git.v4/plumbing"
)

// See https://github.com/jelmer/dulwich/blob/master/dulwich/pack.py and
// https://github.com/tarruda/node-git-core/blob/master/src/js/delta.js
// for more info

const (
	// Standard chunk size used to generate fingerprints
	s = 16

	// https://github.com/git/git/blob/f7466e94375b3be27f229c78873f0acf8301c0a5/diff-delta.c#L428
	// Max size of a copy operation (64KB)
	maxCopySize = 64 * 1024
)

// GetDelta returns an EncodedObject of type OFSDeltaObject. Base and Target object,
// will be loaded into memory to be able to create the delta object.
// To generate target again, you will need the obtained object and "base" one.
// Error will be returned if base or target object cannot be read.
func GetDelta(base, target plumbing.EncodedObject) (plumbing.EncodedObject, error) {
	return getDelta(new(deltaIndex), base, target)
}

func getDelta(index *deltaIndex, base, target plumbing.EncodedObject) (plumbing.EncodedObject, error) {
	br, err := base.Reader()
	if err != nil {
		return nil, err
	}
	defer br.Close()
	tr, err := target.Reader()
	if err != nil {
		return nil, err
	}
	defer tr.Close()

	bb := bufPool.Get().(*bytes.Buffer)
	bb.Reset()
	defer bufPool.Put(bb)

	_, err = bb.ReadFrom(br)
	if err != nil {
		return nil, err
	}

	tb := bufPool.Get().(*bytes.Buffer)
	tb.Reset()
	defer bufPool.Put(tb)

	_, err = tb.ReadFrom(tr)
	if err != nil {
		return nil, err
	}

	db := diffDelta(index, bb.Bytes(), tb.Bytes())
	delta := &plumbing.MemoryObject{}
	_, err = delta.Write(db)
	if err != nil {
		return nil, err
	}

	delta.SetSize(int64(len(db)))
	delta.SetType(plumbing.OFSDeltaObject)

	return delta, nil
}

// DiffDelta returns the delta that transforms src into tgt.
func DiffDelta(src, tgt []byte) []byte {
	return diffDelta(new(deltaIndex), src, tgt)
}

func diffDelta(index *deltaIndex, src []byte, tgt []byte) []byte {
	buf := bufPool.Get().(*bytes.Buffer)
	buf.Reset()
	buf.Write(deltaEncodeSize(len(src)))
	buf.Write(deltaEncodeSize(len(tgt)))

	if len(index.entries) == 0 {
		index.init(src)
	}

	ibuf := bufPool.Get().(*bytes.Buffer)
	ibuf.Reset()
	for i := 0; i < len(tgt); i++ {
		offset, l := index.findMatch(src, tgt, i)

		if l == 0 {
			// couldn't find a match, just write the current byte and continue
			ibuf.WriteByte(tgt[i])
		} else if l < 0 {
			// src is less than blksz, copy the rest of the target to avoid
			// calls to findMatch
			for ; i < len(tgt); i++ {
				ibuf.WriteByte(tgt[i])
			}
		} else if l < s {
			// remaining target is less than blksz, copy what's left of it
			// and avoid calls to findMatch
			for j := i; j < i+l; j++ {
				ibuf.WriteByte(tgt[j])
			}
			i += l - 1
		} else {
			encodeInsertOperation(ibuf, buf)

			rl := l
			aOffset := offset
			for rl > 0 {
				if rl < maxCopySize {
					buf.Write(encodeCopyOperation(aOffset, rl))
					break
				}

				buf.Write(encodeCopyOperation(aOffset, maxCopySize))
				rl -= maxCopySize
				aOffset += maxCopySize
			}

			i += l - 1
		}
	}

	encodeInsertOperation(ibuf, buf)
	bytes := buf.Bytes()

	bufPool.Put(buf)
	bufPool.Put(ibuf)

	return bytes
}

func encodeInsertOperation(ibuf, buf *bytes.Buffer) {
	if ibuf.Len() == 0 {
		return
	}

	b := ibuf.Bytes()
	s := ibuf.Len()
	o := 0
	for {
		if s <= 127 {
			break
		}
		buf.WriteByte(byte(127))
		buf.Write(b[o : o+127])
		s -= 127
		o += 127
	}
	buf.WriteByte(byte(s))
	buf.Write(b[o : o+s])

	ibuf.Reset()
}

func deltaEncodeSize(size int) []byte {
	var ret []byte
	c := size & 0x7f
	size >>= 7
	for {
		if size == 0 {
			break
		}

		ret = append(ret, byte(c|0x80))
		c = size & 0x7f
		size >>= 7
	}
	ret = append(ret, byte(c))

	return ret
}

func encodeCopyOperation(offset, length int) []byte {
	code := 0x80
	var opcodes []byte

	var i uint
	for i = 0; i < 4; i++ {
		f := 0xff << (i * 8)
		if offset&f != 0 {
			opcodes = append(opcodes, byte(offset&f>>(i*8)))
			code |= 0x01 << i
		}
	}

	for i = 0; i < 3; i++ {
		f := 0xff << (i * 8)
		if length&f != 0 {
			opcodes = append(opcodes, byte(length&f>>(i*8)))
			code |= 0x10 << i
		}
	}

	return append([]byte{byte(code)}, opcodes...)
}